The present invention relates generally to an oxygen sensor for detecting the oxygen concentration of a mesurement gas, especially an exhaust gas produced by an internal combustion engine. More particularly, the invention is concerned with such an oxygen sensor which has a planar elongate sensing element, and a protective covering which encloses the sensing element to protect the same against direct exposure to the measurement gas.
An oxygen sensor has been known, which detects or determines the oxygen concentration of an exhaust gas emitted from internal combustion engines, for the purpose of controlling the cmbustion or fuel burning condition of the engine according to signals produced by the oxygen sensor, and thereby purifying the exhaust gas and saving the fuel consumption of the engine. An example of such oxygen sensor uses a sensing element which comprises a body of oxygen-ion conductive solid electrolyte such as zirconium oxide doped with calcium oxide or yttrium oxide, and further comprises suitable electrodes disposed on opposite surfaces of the solid electrolyte body. In this oxygen sensor, one of the electrodes is exposed to a reference gas while the other electrode is exposed to the exhaust gas. In operation, the oxygen sensor produces an output signal which represents an electromotive force induced between the two electrtodes according to the principle of an oxygen concentration cell. In recent years, there has been an increasing trend to use a planar elongate sensing element rather than a conventionally used tubular sensing element, in view of ease of manufacture and structural simplicity of the sensor. In an oxygen sensor using such a planar elongate sensing element, an oxygen detecting member to be exposed to an exhaust gas or other mesurement gas, is disposed in an oxygen detecting portion of the sensing element, which portion is located adjacent to one end of the elongate sensing element. Another type of oxygen sensor is known in the art, which uses a sensing element including an elongate plate on which is provided an oxide such as titanium oxide, whose electrical resistance is varied in relation to the oxygen concentration of the exhaust gas or other measurement gas to which the sensing element is exposed. In operation, the oxygen sensor detects a variation in the electrical resistance of such oxide which represents the oxygen partial pressure of the measurement gas.
Oxygen sensors of various types as introduced above are installed such that their sensing element is partially inserted into a conduit through which an exhaust gas or other measurement gas is caused to flow, whereby the detecting portion of the sensing element, more precisely, the detecting member located at a surface of the sensing element, is exposed to the measurement gas. Generally, the oxygen sensor employs a protective covering member, usually of cylindrical shape in cross section, in order to enclose the sensing element for various purposes which include: to protect the element against thermal shock by high-temperature exhaust gas; to lower the temperature of the exhaust gas before it reaches the sensing element; and to prevent particles in the exhaust gas from being deposited on the sensing element.
The protective covering member has fluid-flow apertures in its cylindrical wall, so that the measurement gas flowing through the conduit is introduced into the protective covering member and brought into contact with the sensing element accommodated in the covering member. In this connection, it is required that the amount of flow of the measurement gas into the covering member for contact with the sensing element be held sufficient for detection of the oxygen concentration, but held to a minimum for protecting the sensing element. Namely, an excessive amount of flow of the gas into the covering member will result in early deterioration of the sensing element, i.e., shortened service life of the sensing element.
In a common oxygen sensor using a planar sensing element as previously indicated, a detecting member provided in a detecting portion of the sensing element in the vicinity of its one end is disposed in a very limited area of one of the opposite surfaces of the planar sensing element. This means that only a small amount of flow of the measurement gas into the protective covering member is required to accomplish the measurement of the oxygen concentration. In addition, the sensing element, more particularly its detecting member disposed in the small area is easily deteriorated if the structure of the protective covering member allows the measurement gas to flow directly against the detecting member.
In the known oxygen sensor using a planar sensing element, the sensing element is enclosed by a cylindrical protective covering member which has the same construction as a protective covering member used for a conventional tubular sensing element. Stated in more detail, the protective covering member enclosing the planar sensing element has fluid-flow apertures or openings over the entire circumference of its cylindrical wall. As a result, a large amount of the measurement gas, e.g., an exhaust gas flowing through an exhaust pipe, is introduced into the protective covering member such that not only the detecting member at the detecting portion at the end of the sensing element but also other portions of the element not carrying an electrode are directly exposed to streams of the gas. Consequently, various components of the oxygen sensing element such as a solid electrolyte body and platinum electrodes are unnecessarily deteriorated, and the life of the oxygen sensor is accordingly shortened. In addition to these problems, the known oxygen sensor of the type using a planar sensing element suffers another problem that the response of the sensor is affected by the position of the fluid-flow apertures or openings formed in the protective covering member, and by the circumferential position of the covering member with respect to the line of flow of the measurement gas through the conduit to which the sensor is attached.